Troubleshooting Brown Leaf Tips on Apple Seedling

Apple seedling is 7cm tall with 5 leaves, stopped growing, one leaf has brown tip. Only one germinated. How to save it?

Seedling Care and Stratification Context

Apple seeds need 90-180 days below 40°F to germinate well (why only one sprouted). Simulate: pot seeds, seal in bag, refrigerate 90-180 days, then warm. For your seedling: pot up to larger container or plant outside if no more frost. Water only when top soil dry. Light fertilize with half-strength or fish emulsion. Brown tip = likely watering issue (too much or too little) or low humidity.